It was a solid episode. Not great, not awful but I can't muster any complaints other than perhaps being slightly wonky in pacing. I stress this, I mean only slightly, not even sure I'd say it worthy of note. The early part was my favorite do to the flashbacks and another appearance of unicorn Twilight! (of which I feel conflicted as whether to consider the best Twilight, but that's another story). The moral was surprisingly strong as well.
Perhaps I could see someponies seeing it as a little lackluster, but for me it was a solid 8/10. This is one of those episodes that I feel could grown on me later and will at least stay average if it doesn't.
Now for some other observations:
1: G1 Star Swirl? What kind of toyline from in universe... My Little Wizards?
2: Twilight sure lacks knowledge on the Canterlot library's inner workings despite being Princess Celestia's student and spending hours there. Some things I can see; like her not ever knowing of the return fees not stacking past a month. But her not knowing that Dusty Pages is gone despite having dropped by both Canterlot and the Library seems rather strange. Though this is consistent from all the way back from her not knowing of the Star Swirl the bearded section in It's About Time. You can't chalk it up to new writers messing it up, it's actually them keeping consistency in a weird sort of way. If maybe accidentally.
3: I liked the flashback, kind of nostalgic. Need to check if they simply reused animation or reanimated it any.
4: What where some of the student 6 doing in that random part of equestria?
5: Mooncurve's Seven Theories on Bending Time? 90% sure it's just a random book but you never no if it could be some cameo for the endgame. Just wanted to note it.
